1. Field of the Invention
The present invention relates generally to a code division multiple access mobile communication system, and more particularly to a method for minimizing searching time for a transport format selection in a code division multiple access mobile communication system.
2. Description of the Related Art
In general, code division multiple access communication (CDMA) systems can be classified into synchronous systems and asynchronous systems. The asynchronous systems include a Universal Mobile Terrestrial System (hereinafter, referred to as “UMTS”).
FIG. 1 schematically illustrates a layer structure of a conventional CDMA mobile communication system. Referring to FIG. 1, a Radio Resource Control (hereinafter, referred to as “RRC”) layer 111 transmits a control message for transport format selection to a Medium Access Control (hereinafter, referred to as “MAC”) layer 115. In this case, the RRC layer 111 transmits the control message for the transport format selection and also a plurality of control messages for controlling the operation of the MAC layer 115. Further, a Radio Link Control (hereinafter, referred to as “RLC”) layer 113 receives a Service Data Unit (SDU) from a higher layer and compares the received SDU with a Protocol Data Unit (PDU). When the received SDU is smaller than the PDU, the RLC layer 113 concatenates the received SDU with other SDUs, so as to generate a PDU having a suitable size. In contrast, when the received SDU is larger than the PDU, the RLC layer 113 segments the received SDU, to generate a PDU having a suitable size. Further, the RLC layer 113 transfers the generated PDUs to the MAC layer 115 through a logical channel.
The UMTS channels can be classified into physical channels, transport channels, and logical channels. The physical channels include downlink channels such as a Physical Downlink Shared Channel (PDSCH), a Dedicated Physical Control Channel (DPCCH), and a Dedicated Physical Data Channel (DPDCH), and uplink channels such as a Dedicated Physical Channel (DPCH). The logical channels can be represented by Dedicated Channels (DCHs), which include a Dedicated Control Channel (DCCH) and a Dedicated Traffic Channel (DTCH). The transport channels include a Random Access Channel (RACH) and a Common Packet Channel (CPCH).
The MAC layer 115 receives a Transport Block Set (TBS) from the physical layer (PHY) 117, divides the received TBS into Transport Blocks (TBs), converts the divided TBs into PDUs, and transfers the PDUs to the RLC layer 113. Then, the RLC layer 113 converts the received PDUs into SDUs and transfers the SDUs to the higher layer. In contrast, the MAC layer 115 receives a PDU from the RLC layer 113, divides the received PDU into TBs, which are real units transmitted through the transport channel, and transfers the TBs to the physical layer 117. The physical layer 117 converts the TBs received from the MAC layer 115 into radio frames, which are real units transmitted from the physical layer, and transmits the radio frames over the air through a corresponding physical channel.
Primitives are utilized in data transmission between the layers described above, that is, the RRC layer 111, the RLC layer 113, and the physical layer 117. Buffers for storing data, such as a shared memory, are interposed between the MAC layer 115 and the RLC layer 113 and/or between the MAC layer 115 and the physical layer 117. In other words, the RLC layer 113 converts the SDUs received from the higher layer into the PDUs, buffers the PDUs in a Dedicated Control Channel/Dedicated Transport Channel (DCCH/DTCH) buffer 119, and reports the buffering to the MAC layer 115 through primitives. Further, whenever the PDUs must be read, the MAC layer 115 reads the PDUs stored in the DCCH/DTCH buffer 119 and maps them onto the transport channel. In other words, according to necessities or when the MAC layer 115 receives primitives from other layers, the MAC layer 115 reads the PDUs stored in the DCCH/DTCH buffer 119 and maps them onto the transport channel, generates TBs by multiplexing and adding headers of the MAC layer 115 according to the type of the mapped transport channel, and transmits data to L1 (Layer 1) for the transport channel. Further, the MAC layer 115 buffers the generated TBs into the transport channel buffer 121. When the TBs must be transmitted, the physical layer 117 reads and transmits the TBs stored in the transport channel buffer 121.
Hereinafter, TBs transmitted through the same single transport channel during one Transmission Time Interval (hereinafter, referred to as “TTI”) will be called a “Transport Block Set (TBS)”, the number of bits in each TB of the TBS will be called a “transport block size”, and the number of the TBs constituting the TBS will be called a “Transport Block Set Size (TBSS)”. In this case, a node B reports the TBSS to a User Equipment (hereinafter, referred to as “UE”), so that the number of bits rate-matched in a physical layer of the UE can be estimated. In this case, the rate matching scheme is information indicating the type in which repetition or puncturing has been performed when the physical layer of the UE has repeated or punctured UE data. Further, as described above, the UE can simultaneously set a plurality of transport channels corresponding to its transmission characteristics (for example, transport channels capable of providing various error correction functions). Each of the transport channels may be utilized in transmitting an information stream of one radio bearer or in transmitting L2 (Layer 2) and a higher layer signaling message. Mapping and transmitting the transport channels onto and through the same or different physical channels are implemented by the physical channel mapping operation of the physical channel 117.
The characteristics of the transport channels are determined according to the channel coding scheme employed in the transport channel, e.g., a convolutional coding scheme, and the Transport Format (TF) or the Transport Format Set (TFS), which define processing in the physical layer, e.g., interleaving and service-specific rate matching. In other words, the transport format implies the set whose members are data processing schemes of the physical layer for the transport channel, and the transport channel usually defines the coding rate and the channel coding scheme in which the data transmitted through the corresponding transport channel have been coded, the size (transport block size) by which the data are divided and transmitted, and the number of TBs that can be transmitted during one TTI. Further, the timing of the TBs is exactly fixed to the frame timing of the physical layer 117, that is L1 (Layer 1). For example, the TB is generated at every 10 ms, that is, at every point of time which corresponds to a product obtained by multiplying 10 ms by an integer. Therefore, two different transport channels have different details in relation to the transport channels, which means different transport formats.
The transport format can be divided into two parts including a dynamic part and a semi-static part, as shown in Table 1.
TABLE 1Transport Format typeAttributesDynamicTransport Block SizeTBS sizeSemi-staticTTIError protection schemeType of error protection, turbocode, convolutional code orchannel codingCoding rateSize of CRC
In Table 1, the dynamic part includes information about a transport block size and a TBSS. The semi-static part includes information about TTI, a size of a Cyclic Redundancy Check (CRC), and an error protection scheme, which includes coding rate and channel coding scheme for error protection. Further, as described above, a transport format is assigned to each of the transport channels according to the characteristics of the mapped physical channel. In this case, the Transport Format Set (TFS) is a set whose members are all transport formats that can be assigned to the transport channels, and the Transport Format Indicator (TFI) is an identifier for identifying each element constituting the transport format set, that is, each of the transport formats. In this case, semi-static parts of all the transport formats are equal to the semi-static parts in the transport format set. Further, the transport block size and the TBSS information contained in the dynamic part are generated corresponding to the bit rate of the transport channel. Therefore, when the bit rate of the transport channel changes according to channel environments and/or service types, only the TBSS or both of the TBS and the TBSS can be changed. In this case, when the transmission rate of the transport channel is fixed or slowly changes, the transport format is mapped to the transport channel. In contrast, when the transmission rate of the transport channel rapidly changes, the transport format set is mapped to the transport channel.
Further, Transport Format Combination (TFC) indicates a combination of the transport formats transmitted to the physical layer 117 through a Coded Composite Transport Channel (CCTrCH) of the UE, which has one transport format for each transport channel, and Transport Format Combination Set (TFCS) indicates a set of the TFCs transmitted through the CCTrCH. In this case, the TFCS needs not include all TFCs of the corresponding transport channels. Further, since a plurality of TFCSs are generated, Transport Format Combination Indicators (TFCIs) are required to identify the TFCI being currently assigned to the transport channel. Therefore, when a transmitting-side of the communication entity, e.g., a node B, transmits a transport channel with a TFCI corresponding and mapped to the transport channel, a receiving-side of the communication entity, e.g., a UE, can decode and demultiplex the transport channel by analyzing the TFCI of the transport channel.
Further, since a plurality of transport channels can be time-division-multiplexed through the same physical channel, the UE should be capable of recognizing the transport channel to which the physical channel received at a predetermined point of time pertains. Therefore, the UE provides an indicator to each of the transport channels in order to differentiate and identify the transport channels. This indicator is the Transport Channel Indicator (TCI).
Whenever the RLC layer 113 transmits a data request signal, the RRC layer 111 transmits a control signal for selecting a transport format assigned to the transport channel construction to the MAC layer 115. The RRC layer 111 assigns priority values, for example, ‘1’ to ‘8’, to a plurality of logical channels, for example, 8 logical channels, between the RLC layer 113 and the MAC layer 115, to control scheduling of uplink data. In this case, from among the priority values, ‘1’ is a value having the highest priority and ‘8’ is a value having the lowest priority. The selection of TFCs in the UE depends on the priorities assigned to the logical channels by the RRC layer 111. Whenever the RLC layer 113 transmits a data request signal, the MAC layer 115 selects a proper transport format for data transmission under the control of the RRC layer 111. Further, in the transmission according to the priorities, some TBs from among the TBs of each of the logical channels may be interrupted and delayed for data transmission of another logical channel having the next-highest priority. In this case, this interruption of TBs for data transmission of another logical channel is also implemented under the control of the RRC layer 111, and the priority of the interrupted TBs is set to be ‘0’ which is higher than the highest priority ‘1’, so that the data having the priority of ‘0’ can be transmitted prior to any other TBs.
Further, when the UE transmit power approaches the maximum transmit power that can be transmitted by the UE, and the internal loop for power control cannot be maintained any more due to the coverage problem, the UE assigns a transport format combination having a bit rate lower than that of the current transport format combination to the transport channel. When a bit rate of a logical channel that transfers data from a CODEC supporting the variable rate operation conflicts with the lower bit rate, the bit rate of the CODEC is changed to avoid the conflict. Further, the UE continuously measures whether the maximum transmit power of the UE can support the temporarily interrupted transport format combination. When the maximum transmit power of the UE can support the temporarily interrupted transport format combination, transport combinations are assigned to the transport channels in reconsideration of the temporarily interrupted transport format combination.
As described above, the MAC layer 115 performs a transport format selection in response only to the data transmission request of the RLC layer 113, has a transport format table including all transport formats which can be assigned for the transport format selection, and searches the transport format table under the control of the RRC layer 111 when data transmission is requested by the RLC layer 113, to select a transport format for the corresponding transport channel. However, searching the transport format table, which includes transport formats of all cases, in order to assign a transport format to one transport channel requires relatively too much time in the transport format selection and may cause an overload due to the transport format selection.